Pat - 
I'm not a classicist, just a hacker who loves classical languages. I 
personally like Goodwin and Gulick for a grammar. I've taken Homeric and Attic 
greek, and studied koine Greek on my own, and I've found G&G up to the task, 
whatever the form of Greek used. It is well-organized, with many examples, so 
when you're confronted with a strange construction, it's not too difficult to 
find the 'strange Greek rule number' which applies to the circumstance. 
If you only wish to study koine Greek, perhaps a koine-specific grammar is the 
way to go. If you fall in love with Greek, as many do, I recommend a more 
encompassing Greek grammar such as G&G. 
Mary Ann Davidson